Donald Trump and Hillary Clinton face key primary elections
Republican Donald Trump and Democrat Hillary Clinton could emerge from primary elections in five US states as presumptive presidential nominees, setting in motion a one-on-one race in the most chaotic American political contest in years.
Mr Trump entered the voting embroiled in one of the biggest controversies of his contentious campaign. The billionaire property baron has urged supporters to physically confront protesters at his campaign events and now faces criticism for encouraging violence after skirmishes broke out at a rally last week in Chicago.
Florida and Ohio are Tuesday's biggest prizes but Missouri, Illinois and North Carolina are also awarding a large cache of delegates to the parties' national nominating conventions.
While Mr Trump could take all five states, that would still not assure he goes into the Republican convention this summer with the needed majority of delegates. His closest competition has come from Texas senator Ted Cruz, who has won in seven states.
Mr Trump must also overcome Florida senator Marco Rubio and Ohio governor John Kasich, who are battling for their political lives in their home states.
Mrs Clinton, once seen as the clear favourite for the Democratic nomination, still faces a challenge from Vermont senator Bernie Sanders, who has shown surprising staying power after pulling off an upset victory in the industrial state of Michigan and is trying to build momentum in the Mid West.
If Mrs Clinton comes out of Tuesday's contests with decisive wins in several states, it would be difficult for Mr Sanders to catch her because all the state Democratic nominating contests allot their delegates proportionally. Tuesday's voting has 691 delegates at stake for the Democrats.
Mr Trump holds a comfortable lead in the Republican delegate count. If he sweeps Tuesday's contests, he would cross an important threshold by collecting more than 50% of the delegates awarded so far.
While polls show Mr Trump leading Mr Rubio in Florida's winner-take-all contest for 99 delegates, Mr Kasich holds a narrow lead in Ohio, where 66 delegates are at stake.
That makes Ohio the key state in determining whether Mr Trump puts himself on a path to winning the nomination by the end of the primary season on July 7. A loss there means the race could result in a contested convention.
The violent atmosphere at some Trump events has deepened concern over his candidacy in some Republican circles. Mr Rubio and Mr Kasich have suggested they might not be able to support Mr Trump if he is the nominee, an extraordinary stance for intraparty rivals.
Mr Kasich, who has been restrained in his criticism, said he would be "forced, going forward, to talk about some of the deep concerns" he has about Mr Trump's campaign.
Mr Trump has denied playing any role in encouraging violence against protesters.
Mr Trump won easily in the Northern Mariana territorial caucus on Tuesday, picking up nine delegates. That gave him 469 to 370 for Mr Cruz, 163 for Mr Rubio and 63 for Mr Kasich. It takes 1,237 to win the Republican nomination.
Mrs Clinton headed into Tuesday's primary with 768 pledged delegates compared with 554 for Mr Sanders, according to a count by the Associated Press. Including superdelegates, elected officials and party insiders who are free to choose any candidate, Mrs Clinton has 1,235 total delegates, more than half the amount needed to clinch the nomination, while Mr Sanders has 580.
For the Democrats, 2,383 delegates are needed to win the nomination.